Firstly, welcome to Eorzea!

Adding to what Cana said, tanking really does put you right into the action in that it allows you to guide the flow of the party - most parties will usually wait til you make the move before engaging (of course, there are always exceptions to that), and it is incredibly satisfying to get through a challenging battle or dungeon where you've been the guiding light to that success.

Of course the other side of that coin is not to be arrogant or selfish - a good tank is only as good as his teammates, especially the healer but even the DPS are just as important too, as they're the ones keeping you alive while you keep the horrid Voidsent from eating the squishy lalafell WHM's face - so good communication is key.

But really, if you're especially wanting to play as PLD, then you picked a good class to get yourself aquainted with the game, as PLD's starting class is GLA which is a good beginner's tank, as it's abilities start off being simple to learn the ropes with and there are no complex mechanics to keep your alive or hold hate with (bare in mind all classes and Jobs underwent revision with the new expansion so a lot of things changed with GLA and PLD especially). So really, if you're interested in taking up the sword and shield, give GLA a try for yourself! The beauty of this game is like many FF games, you're not stuck in the same class forever, you can change classes/Jobs at will by simply equipping the weapon or tool. So even if you try GLA and don't like it, don't feel you've made a big mistake. Find another class or Job that's right for you!